1. Field of the invention
This invention relates to a method for the manufacture of pyrolytic graphite with high crystallinity.
It also relates to electrodes of a rechargeable battery, wherein an active material for the electrode is pyrolytic graphite with high crystallinity mentioned above, into which alkali metals, alkaline earth metals, rare earth metals, or transition metals are intercalated as an electron-donor type dopant, or halogens, halogen compounds, or oxygen acids are as an electron-acceptor type dopant.
2. Description of the prior art
In recent years, attention has been paid to secondary batteries that use an alkali metal such as lithium as an anode, with the advance of microtechnology and request for power economization in electric appliances. However, the practical application of a simple metal to secondary batteries is very difficult. A simple metal used for the electrodes, after only a couple of cycles of electrical charge-discharge, grows dendrite which causes internal shortcircuits. On the other hand, materials such as alloys with low melting points, and organic materials have been found to be able to be doped or undoped more efficiently with metal atoms such as lithium. Especially, the development of secondary batteries that make use of the electrochemical introduction of dopant between the layers of compounds with a layered structure such as graphite is progressing with special vigor.
In particular, since graphite can be introduced with both electron-donating substances and electron-accepting substances. Graphite is expected as an excellent material for use as the electrode material of secondary batteries. However, graphite is ordinarily in the form of a powder, film, foil, fiber, etc., it is difficult to form the desired shape of electrode, and also, a complicated procedure is needed to fix these kinds of materials on the substrate of the electrode, which will act as the current-collector. In these cases, a binding agent, an electric conductive material, etc., are needed as supplementary materials to form an electrode and so there are the disadvantages that the capacity per unit weight or unit volume will decrease.
It may be possible to deposit pyrolytic carbon on a conductive substrate of aluminium, copper, etc., by the chemical vapor-deposition (CVD) method, etc., so as to form an electrode. However, such carbon deposits are only slightly graphitized, so it is not possible to solve the problems that are described above. Because, for the synthesis of graphite, in general, a long period of treatment in a manufacturing step at high temperature and high pressure is needed. For example, when methane is the starting material, it is decomposed at about 2000.degree. C. or more, and in addition, heat treatment at a high temperature of near 3500.degree. C. and high pressure is used for the purpose of achieving graphitization. However, no conductive electrode substrates for batteries exist that can withstand such high temperatures.
As a method for producing a pyrolytic carbon at low temperatures, U.S. Pat. No. 3,769,084 to T. Saito and T. Gejyo discloses a method for forming a carbon coating on a metal substrate by thermally decomposing a hydrocarbon at a temperature lower than 800.degree. C. In this method, the metal substrate has an amorphous nickel layer plated thereon. The nickel layer acts as a catalyst for thermal decomposition of hydrocarbons, and the thermal decomposition reaction proceeds at low temperatures.
However, the density of the carbon coating obtained by the method mentioned above is at most 1.9 g/cm.sup.3. It is well known in the art that crystalline graphites have a density of 2.25 g/cm.sup.3. Therefore, the carbon coating is not composed of a pyrolytic graphite with high crystallinity, which indicates that the amorphous nickel layer cannot promote graphitization of the carbon coating formed thereon.
On the other hand, there has been another method to prepare pyrolytic carbon at relatively low temperatures, utilizing dehydrogenation reactions, dehydrohalogenation reactions, decarboxylation reactions, dehydration reactions of selected organic compounds as the starting materials. However, there are no examples of carbon deposit with high crystallinity having been achieved. To make use of the structural anisotropy of graphite for the host layered material to be intercalated with lithium atoms, it is necessary to establish crystalline graphite at low temperatures.